Evaporators or plate-finned coil heat exchangers typically comprise a bundle of numerous lengths of pipe or tubing in a square or staggered array, with numerous plate fins slid over and cross-sectionally surrounding the tubes. The plate fins have holes punched in them to correspond to the tube array geometry. In the finished product, a fan or blower causes air to flow parallel with respect to the fins and perpendicular with respect to the tubes.
Usually, the fins have a formed collar at each hole that causes the tube extending therethrough to fit securely and snugly into the fin. The collar allows the fin to remain in good thermal contact with the tube, thereby providing good heat transfer into or out of the tube. Typically, the ends of the tubes are fitted with return bends to form at least one series of tubes. The ends of each series of tubes are fitted to inlet and outlet headers to complete the closure of the heat exchanger.
The tubes, bends, and fins are constructed of steel, copper, aluminum or other suitable metals and alloys. Typically, for steel construction, the tubes, bends, and fins are fabricated into a coil assembly, and then the coil assembly is hot dip galvanized. The galvanizing improves the corrosion resistance of the steel and also thermally and mechanically bonds the fin to the tube. For copper or aluminum construction, where galvanizing is not used, the tubes are expanded into tight contact with the fins. Such expansion is achieved by forcing an oversized mandrel through the individual tubes, or by hydraulically pressurizing the coil assembly.
Numerous factors enter into the geometry of the tube/fin arrays. The two most important factors are the efficiency of the heat transfer surface (the area in contact with the air flow) and the amount of resistance to air flow through the tube bundle (measured in terms of pressure drop).
The heat transfer process in the coil assembly involves numerous steps. First, a refrigerant or other heat exchange fluid is caused to boil or to condense on the inside surface of the tubes through well known methods. Boiling or condensing refrigerant flowing through tubes is a very turbulent, active and efficient mode of heat transfer. A typical heat transfer coefficient might be 400 BTU/hr-ft.sup.2 -degree F. (2270 W/m.sup.2 -K).
Next, the heat is conducted through the walls of tubes. The tube wall is relatively thin and the conductivity of most metals is known to be high. For 0.060 inch (1.5 mm) thick steel tube, the conduction coefficient would be around 5200 BTU/hr-ft.sup.2 -degree F. (29,500 W/m.sup.2 -K). Finally, the heat is transferred by conduction from the tube surface to the air. Due to the physical properties of air, the heat transfer coefficient from a bare tube to the air is around 15 BTU/hr-ft.sup.2 -degree F. (85 W/m.sup.2 -K).
Plainly, the final step in the transfer is the limiting factor, and the overall rate of heat transfer can never be greater than the outside coefficient. Thus, the external heat transfer coefficient must be improved in order to improve the overall heat transfer coefficient.
As is well known, the external heat transfer may be increased by moving the air past the tubes. The air must be turbulent enough to prevent streamline flow through the coil. That is to say, all the air going through the coil must come into contact with one or more of the tube surfaces for as long and as often as possible before leaving the coils. If air, due to the geometry of the tube bundle, is allowed to pass through the coil assembly without coming into contact with the tube (bypass air), then the effort expended (fan horsepower) to move the bypass air has been wasted.
As a way to improve coil bundle performance, more tubes can be added to the bundle. Thus, tube surface area is increased and bypass air is decreased. However, additional tube surface requires more expense. Also, the tubes require considerable space in the coil array. If too many tubes are stacked together too tightly, airflow will be restricted to the point that more fan horsepower is required. Moreover, and as a practical limitation on tube density, moving tubes closer together requires return bends with tight radii. Such return bends are not easily fabricated, and welding such return bends to the ends of the tubes is exceedingly difficult.
As is well known, the addition of fins to the coil assembly greatly increases the heat transfer area of the coil assembly and accordingly enhances the external heat transfer process. In particular, by increasing the external surface area of the coil assembly by a factor of 10, as is typical, much more area is in contact with the air stream. Although adding fins to the spaces between the tubes increases airflow resistance, the fins are very thin material (about 0.005 to 0.02 inch {0.13-0.5 mm} thick) and are aligned in a direction generally parallel with respect to the air flow. Thus, the benefit of the fins far outweighs the airflow resistance and fan horsepower penalties. Typically, the spacing between fins is from about 0.16 to 0.33 inch (about 4.1 to about 8.4 mm).
Fin efficiency is, at best, always somewhat less than the tube surface efficiency because the fin is physically (and thermally) extended from the refrigerant inside the tube. Adding a fin adds a fourth step to the heat transfer process described above, in that heat must first pass through the tube and then to the fin. Although the fin is very conductive, the thin material provides limited heat conduction. Thus, as the perimeter of the fin gets farther away from the tube, the efficiency of the fin decreases. However, the efficiency of the fin can be somewhat enhanced with ripples, wrinkles and bumps. These features improve the heat transfer from the surface of the metal to the air by increasing the fin surface area, increasing turbulence and reducing air bypass. However, these features also increase the pressure drop of the air, so that a tradeoff must be considered in addition to these features.
Since fin efficiency falls off with increasing radial distance from a tube, tube geometry and spacing becomes even more important. On the one hand, moving tubes closer together raises the efficiency of the fin surfaces in between the tubes. On the other hand, moving tubes closer together also increases tube density in the bundle. As previously stated, higher tube density requires higher fan horsepower due to the restricted air flow. Thus, within the limits of tube cost, manufacturing capabilities and air flow restrictions, the more tubes, the better for optimum coil efficiency.
The number of compromises and tradeoffs in finned coil design are numerous. All are aimed at maximizing the efficiency of the external heat transfer, minimizing air flow resistance and minimizing material costs.
Some of the existing designs in the art of heat exchanger coil assemblies are as follows:
Rectangular tube spacing: By arranging tubes in straight rows and columns, numerous advantages are obtained from the relative simplicity of the arrangement. However, such an arrangement allows for a relatively high amount of bypass air. Another problem arises in that, except for the air side tube, each tube in a column is directly in the "shadow" of another tube, and does not receive an adequate flow of air. As a result, the most important portions of the fins, which are closest to the tubes, are in the "shadows" and do not receive adequate air flow, either.
Triangular or staggered tube spacing: By arranging tubes in a triangular pattern, with transversely oriented rows of tubes staggered, the tubes can be much closer together while still maintaining a good open area percentage for airflow through the coil. In a typical equilateral spacing of 2.5 inches (63.5 mm) between tubes having 1 inch (25.4 mm) diameter, the open area at any row of the coil (1 row % open) is 60%. Also, the air passing through the coil is forced to go over and around each succeeding column of tubes. When a second staggered row is considered in the open area calculation, then the projected open area (2 row % open) nominally becomes only 20%. The nominal 20% open area number is effectively somewhat greater in that the air flow is not as linear as the projection. Regardless, the triangular pattern significantly reduces bypass air without causing high pressure drops, and although tubes are still "shadowed", the increased air turbulence provides better air flow to the "shadowed" spots.
Elliptical tubes: Theoretically, elliptical or compressed tubes offer much less resistance to air flow. Also, elliptical tubes in a bundle may be more tightly spaced while still maintaining a high percentage of open area through the coil. However, return bends connecting the tubes are greatly complicated by the elliptical cross-section to which each return bend must attach, as can be seen in U.S. Pat. No. 3,413,999 (to Thomae). Bending elliptical tubes is exceedingly difficult. As the Thomae patent shows, round tube bends with elliptically stamped ends are known. However, several different return bend configurations are required depending on the angular orientation of the elliptical tubes and the angle that a particular return bend must traverse. Moreover, the return bends of the Thomae patent are extremely limiting in terms of the possible tube geometries. Even more so, each elliptical end portion of the Thomae return tubes is exceedingly difficult to form and provides little room for error.
The present invention overcomes the numerous problems detailed above by providing a coil assembly using elliptical tubes oriented in a plurality of staggered rows, with the major axes of the ellipses alternately rotated from one row to the next at an angle that provides maximum efficiency.
Moreover, the present invention also overcomes the need in such an elliptical tube geometry for several different return bend configurations and provides a coil assembly requiring only one type of return bend. As a result, the configuration of the return bend used to interconnect any two linear tubes is not dependent upon the angle of rotation of the major axis of the ellipse of any of the tubes, nor is it dependent upon the angle that a particular return bend must traverse. Numerous other advantages of the present invention will be evident from the drawings and the description set forth below.